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Name: Monica
Status: student
Grade: 9-12
Location: N/A
Country: N/A
Date: 1/18/2006


Question:
I have read a great deal of books and academic sites on Quantum Mechanics and find it very interesting, particularly where Quantum Entanglement is concerned.

The problem I have with entanglement is that it all seems so simply explained in other ways which do not require 'action at a distance'

What I would like to know is whether there is any experimental data that either proves or strongly suggests that the state of each 'entangled' particle was not already defined from the outset.

For example:

I read an account of a box which emits red light from one side and blue light from the other. These colors are randomly switched. An observer measuring the light from one side can then 'know' the color that a second observer will measure from the other side - Even though this reduces the probability of the second observer getting a particular colour from 0.5 to a definite 1 or 0 it does not mean that any information has been transmitted. Similarly, if I toss a coin and measure the UP side for 'heads' or 'tails' this does not affect or in any way alter a later observation of the DOWN side.

I am sure all these physics theorists are not getting so fired up about something so simple - so I must assume that I am missing some experimental evidence somewhere. Can you point me to the paper or explanation that will help me see this subject as something more than mundane ?

also

I recently heard of a successful experiment in Quantum Cryptography which I had assumed would demonstrate the ability to transfer information via entanglement. What I found was a disappointment since it relies on a second channel of regular communication and entanglement simply allows the sender to know what was sent.

The only real advantage of Quantum Cryptography is the current practical difficulties in reading a particles state without significantly altering it. Being a new science I do not find this current limitation particularly assuring ... far less so, in my view, than conventional mathematical cryptography.

I am not normally such a cynic, honest. As I said before - I am pretty sure I am missing something important here. Can you help me?


Replies:
No, Monica, there is quantum weirdness. You are not missing something. I am not a theoretical physicist, nor do I presume to really understand the phenomenon. However, it is possible to read the experiments done on entanglement and come to the results of those experiments. The results are "facts" they cannot be disputed. Where the paradox emerges is when we try to "explain" the results verbally in terms of pictures and words that really do not apply. It may be, and probably is, the case that we do not have the proper analogs to "explain" the results. In this case what the theoretician has to do is make sure the math is correct and abandon the physical explanation in terms of models that may not be appropriate. Entanglement is a quantum phenomenon, and trying to "explain" the results of the mathematics in terms of macroscopic springs, and balls and stuff that we are familiar with is just not appropriate. All that can be done is let the math take us where it will (assuming it is done correctly). It is a general result of differential equations that if there are two valid solutions to the equation, then any linear combination of those solutions will also be a valid solution. In the case of entanglement what this "means" is that a photon (electron or whatever) that passes through two slits, it actually passes through both slits simultaneously, no matter how far apart the two slits are arranged. There is no classical analog to that behavior. In addition to the book "Entanglement: The Greatest Mystery in Physics" by Amir D. Aczel, the web sites below discuss entanglement. However, the bottom line is the "explanations" are not very convincing in classical terms. Quantum mechanics gives the correct description of the experimental results. The fact that it does not make sense in classical terms is just the limitation of our experience.

http://www.virtuallystrange.net/ufo/updates/2003/oct/m17-006.shtml

http://www.joot.com/dave/writings/articles/entanglement/

http://www.spaceandmotion.com/Physics-Richard-Feynman-QED.htm#Feynman.Double.Slit

Vince Calder

Clarification Point January 2006

Vince Calder's answer to Monica's question (#1770) about quantum entanglement misses the point of her question. Clearly she believes that the experiments show nothing weird at all, and that the physicists appear to be all excited about nothing. His answer that "she is not missing something" and "there is quantum weirdness" contradict each other since she thinks nothing weird is going on.

For example, the EPR paradox about the quantum entanglement of the spin of two electrons DOES seem mundane to an outsider. After all, if you just assume that each electron has an inherent spin that is equal and opposite to the other one, then measuring the spin of one tells you the spin of both electrons. She wants to know, "what is the big deal?"

The accepted quantum explanation is that the electrons' spins were in a superposition of states and that there is a collapse of the wave function which results in an instantaneous change to the wave function everywhere as soon as one of the spins is measured. Indeed, Einstein's whole point with the EPR paradox was that it is more reasonable to believe that the electrons DO already have the appropriate spin quality than to believe in an instantaneous collapse of the wave function - something he called *spooky action at a distance* and which he thought must violate relativity. The fact that quantum mechanics successfully predicts the answer is great. But Monica's question seems to be, "how do you know there is not a simpler explanation, such as, the electrons always had the appropriate inherent spin in the first place (like Einstein believed) so that nothing *weird* is happening at all?"

I suspect the answer she is looking for has something to do with Von Neumann's proof that you cannot supplement quantum mechanics with hidden variables in such a way as to make all of the observables "dispersion free". Bell's theorem would also probably be relevant reading.

Jacob


I do not want to get into a game about quantum weirdness, EPR paradox, or action at a distance. First, it severely challenges my competence in the area. Second, the concept of "entanglement" is mathematically abstract and complicated. I would ask these questions of the responder above: 1. What does the term "an instantaneous change to the wave function" mean? The wave function is a continuous function of time, and consequently it cannot have "an instantaneous change" which implies a discontinuous function. 2. What specifically and mathematically is meant by "a collapse of the wavefunction?". 3. The information of measuring the spin of an electron at point "A" cannot be transmitted to point "B" faster than the speed of light, so the second electron does not "know" whether it is 'up' or 'down' relative to "A", before the measurement of "A" even occurs. 3. The term "dispersion free" is not defined by the responder. 4. I refer the responder to chapter 7 of the book "The Odd Quantum" by Sam Treiman, and still maintain that there is not something weird that is not well understood.

I defer to a quote of Richard Feynman to the effect that anyone who claims to understand quantum mechanics, does not understand the problem.

Vince Calder



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